Although a number of different general types of drill bits are provided for drilling earth formations, the drill bit construction under consideration herein is generally known as a rotary cone type rock bit. A drill bit body structure is provided which is threadedly connectable to drill pipe that is supported and rotated by a drilling rig. The body structure of the drill bit provides leg structures each having stub shafts or axles which provide support for rotary cone type cutter elements that cut away the formation as the drill bit is rotated. Typically, the drill bit is provided with an internal lubrication system having a quantity of fairly viscous lubricant which is retained within the lubrication system by means of sealing elements at each of the rotary cone members. Although virtually all rotary cone type drill bits incorporate lubricant seals, it is also well known that these seals tend to wear rather rapidly because of the harsh abrasive environment within which the drill bit operates. The lubricant seal element of each of the rotary cone cutters of the drill bit is subjected to drilling fluid which contains fine abrasive particulate such as bentonite and drill cuttings eroded from the formation during drilling operations. The drilling fluid, typically known as drilling mud, utilizes water and other liquid materials as a carrier constituent for the solid particulate of the drilling fluid.
A majority of the bearing failures in the drill bits are preceded by failure of the seal which is intended to keep the lubricant inside the bit. Seals are the weakest link in the sealed lubrication systems currently being used in various drill bit designs. A drill bit seal is expected to operate under a very harsh environment which includes abrasive mud, high temperatures and complex movement between the cutter cone and the axis of the shaft that is mounted on. These movements include axial play, radial play, cone wobbling from side to side and a combination of the above. As the bearing wear continues, these radial and axial movements also continue to increase. Associated with these complex cutter movements, it is estimated that pressure fluctuations of high frequency and magnitude reaching .+-.50 psi to .+-.100 psi or even greater are encountered. This is due to the inability of the pressure equalizing system to respond to these rapid fluctuations. A good seal design must have the ability to continue to perform its sealing function under the above movements, pressure fluctuations, high temperature and the abrasive mud environment with a low leakage rate and the design must also provide an extended service life.
Several seal designs have been used by various bit manufacturers which have gradually improved the seal life over the years. One of the more widely used seal designs at present is set forth in U.S. Pat. No. 3,397,927 of Hughes Tool Co. This particular seal design employs an O-ring with a high initial interference to accommodate the cone movements and yet have sufficient compression available to compensate for the high magnitude of abrasive wear that this seal is subjected to. Such a seal operates under "brute force" approach, i.e. using increased seal compression to prolong seal life; it is not designed to operate under any definitive mechanism of hydrodynamic lubrication and therefore allows a direct contact between the elastomeric sealing surface against the relatively rotating metal surface of the cone or shaft, as the case may be. Because of the direct rubbing contact between the seal and the metal surface, abrasive wear of the elastomeric material takes place rather quickly. This problem is further aggravated by the relative axial movement that typically occurs between the seal and its mating surface on the cone. There is a tendency for the abrasive mud particles to be wedged under the sealing interface due to this relative axial movement. This wedging of the mud particles is caused by the gradually converging shape of the O-ring cross-section outside the contact width zone of the sealing surface. The net result of the lack of a lubricant film and the ingress of the abrasive mud particles at the sealing interface is that the elastomeric material of the seal is subject to a high wear rate, thus giving a relatively short seal life. A major percentage of the initial compression of the O-ring seal is thus used up in feeding elastomeric material to compensate for the abrasive wear while maintaining a seal. In order to prolong the seal life, softer elastomers and higher initial compressions have been used.
Another problem associated with such seals is the high rate of heat generation and a localized increase in interfacial temperature of the seal. This is due to high friction which occurs at the dynamic interface in the absence of a lubricant film. High initial interference further aggravates the situation, resulting in further increase in seal temperature. The localized temperatures under the seal interface are thus significantly higher than the ambient environment, which results in a severe reduction of seal life due to the blistering, scorching and hardening of the seal material. Because the temperature generated at the seal interface increases dramatically as the rubbing speed is increased, this seal is not suitable for high bit speeds.
Another design that is described in U.S. Pat. No. 3,765,495 of G. W. Murphy Industries provides competitively equivalent seal life through utilization of a deeper oval cross-section seal to accomplish a larger initial compression of the seal while maintaining a percentage compression ratio of below 10 percent. This seal design also operates with no lubrication at the sealing interface and has well-rounded edges on both the lubricant side and the mud slide which results in wedging of abrasive mud particles in the seal interface due to relative axial motion. The life of this design is also relatively short and comparable to the high interference O-ring seal described earlier.
It is well-known that hydrodynamic lubrication can dramatically improve the life of rotary shaft seals in a clean environment. Prior art shows several designs of elastomeric seals that operate on the basis of hydrodynamic lubrication. By creating a hydrodynamic film thickness of sufficient magnitude, a complete separation between the asperities of the two rubbing surfaces (the elastomeric surface of the seal and the metal surface of the rotating part) can be achieved. This can provide a virtually wear-free seal in a clean, lubricated environment. For example, U.S. Pat. Nos. 3,449,021; 2,867,462; 3,831,954; 2,571,500; 3,195,902; 2,647,770; and 3,272,521 deal with slanted sealing surfaces between tubular members to create hydrodynamic action. U.S. Pat. No. 3,449,021 illustrates such a seal for use between relatively rotating surfaces. Such designs, however, are not suitable for use where an abrasive fluid medium is present as in well drilling applications where the fluid is in intimate contact with the seal due to the high level of agitation. This is due to the tendency of these designs to also promote a strong wedging activity on the mud side, forcing abrasive mud particles from the mud side into the sealing interface which creates very rapid wear due to abrasion and results in early failure of the seal. Thus, even though these designs work successfully in a clean lubricated environment, they are totally unsuitable for conditions where the seal is exposed to the abrasive fluids, such as drilling mud.
The characteristics of lubrication of hydrodynamic seals have been studied by the inventor and reported in several articles. Kalsi, M. S. and G. A. Fazekas, "Feasibility Study of a Slanted `O-ring` as a High Pressure Rotary Seal," ASME Paper No. 72-WA/DE-14 (1972); Kalsi, M. S., "Elastohydrodynamic Lubrication of Offset O-Ring Rotary Seal," ASME Paper No. 80-C2/Lub-7 (1980) and in a dissertation of the same title submitted to the University of Houston in 1975. This study deals with the fundamental lubrication principles for successful use of the hydrodynamic seals in rotary shaft applications. These studies were confined to O-ring cross-section seals operating in clean lubricated environment.
An invention utilizing a slanted seal principle for use in mud motors for drilling applications is disclosed in application Ser. No. 462,464 of Monmohan Singh Kalsi filed Jan. 31, 1983 now U.S. Pat. No. 4,484,753. This invention also employs the principles of hydrodynamic lubrication in a rotary shaft seal and successfully overcomes the application problems that are prevalent in mud motor applications. Specifically, the slanted mud motor seal is designed to operate under high differential pressures utilizing a differential area principle in conjunction with hydrodynamic sealing principles to obtain extended life for the application. It should be pointed out that the use of such a seal in a drill bit application is not technically feasible because of the severe dimensional constraints that must be adhered to. Further, the complexity, bulk and high cost involved with such a seal renders its use in drill bit applications impractical. In drill bit applications, the seal must be very compact, preferably fitting into the dimensional constraint imposed by drill bit designs of the current configuration. Seals of this nature must also provide a very low leakage rate and a simple construction to be practically implemented in drill bits. The drill bit seal operates approximately under zero or relatively low mean differential pressures; therefore, some of the features used in mud motor seals to overcome the difficulties imposed by high differential pressure sealing are not necessary. Also, unlike in mud motor seals described, one cannot rely on a high lubricant leakage rate across the seal to prolong the seal life because of the limited capacity of the pressure equalizing reservoir commonly used in the drill bits. Therefore, this specific application requires a seal that operates under a very low leakage rate, yet prolongs the seal life. Another feature that is required in the drill bit seals to successfully extend their service life is to combat any tendency for ingress of abrasive mud particles into the sealing interface under relative axial motion between the seal and the shaft or cone, as the case may be. This problem does not exist in mud motor seals as the relatively rotating members are substantially fixed in the axial direction with respect to each other by a rigid bearing system. The present invention decribes a seal which successfully overcomes the problems enumerated above for a rotary cone drill bit.